1. Field of the Invention
This invention relates generally to optical scanning systems. More specifically, the invention relates to a coded symbology scanning system which includes a signal processor for pre-processing scanned symbology information and a hardware symbology locator for high-speed locating of potential coded symbology candidates.
2. Description of Related Art
Coded symbologies are being used in an increasingly diverse array of applications. The ability to track a large amount of items quickly and efficiently has lead coded symbologies to be used in applications such as retail checkout, warehousing, inventory control and document tracking. As the volume of items tracked by coded symbologies has increased, the need for optical scanners which operate at high speeds has likewise increased. Many current optical scanners are able to promptly locate and decode a variety of coded symbologies. However, these scanners require high-speed microprocessors and large amounts of accompanying memory, both of which are very expensive.
It has also become common to find more than one coded symbology label affixed to a product. For example, as shown in FIG. 1, a package may have several coded symbology labels which are affixed to the package; such as a manufacturer""s label, a distributor""s label and a retailer""s label. Each coded symbology label may be printed on a different substrate or background. The manufacturer""s label is typically printed as part of the original package. The coded symbology labels of the distributor and retailer, however, may be printed on stickers that are affixed at a later date.
Different coded symbology labels typically have different reflectivity characteristics as normalized by the wavelength of the scanner laser light, i.e., the amount of light that is reflected by the symbology elements. Although some labels may have a flat (i.e., non-shiny) appearance and a lower reflectivity, other labels have a very shiny appearance and a high reflectivity. In order to accurately read all coded symbology labels, it is critical to account for the differences in reflectivity. This requires a scanner with a wide dynamic range.
Most current scanning systems scan a coded symbology by focusing a scanned laser light on a label, detecting light reflected from the label and generating an analog waveform from the reflected light which is representative of the coded symbology. The analog waveform is then converted to binary digital form using an analog to digital converter. Since the transition from analog directly to digital ultimately results in a loss of data, error correction often cannot be adequately performed on an erroneous signal.
High-speed analog to digital converters are typically fixed resolution devices, (e.g. 8 or 10 bit devices), and the number of bits available for encoding the scanned information is fixed. As the dynamic range required for the specific application increases, the resolution of the scanner decreases. This results in limitations when the information is ultimately to be represented in digital form.
Accordingly, there exists a need for a high-speed, efficient coded symbology scanning system.
The present invention comprises a scanner for optically scanning coded symbologies which includes a signal processing unit for pre-processing the scanned symbology information and a hardware symbology locating unit. An analog signal related to the reflectivity of the different portions of the coded symbology is obtained and sampled at the Nyquist frequency, or higher, to retain the information embodied within the signal, and is converted to digital gray-scale. The portion of the analog signal which corresponds to the substrate reflectance (absolute contrast) is removed, and the available dynamic range of the analog to digital converter is used only for the information-bearing portion of the signal (relative contrast). Since all of the decodable information is ultimately encoded in binary form, the information-bearing portion of the signal is retained.
The hardware symbology locating unit includes a plurality of shift registers, each of which derives a value from an input seed which is based upon the width of a detected coded symbology element. The value of a subsequent input seed is compared to the values generated from a prior input seed. Based upon this comparison, the unit determines whether the size of a subsequent symbology element is a ratio match of a prior symbology element. The presence of a potential coded symbology candidate is likely when a plurality of ratio matches are detected.
Accordingly, it is an object of the invention to provide a high-speed symbology locating device which is computationally efficient and does not require a large amount of memory.
Other objects and advantages will become apparent to those skilled in the art after reading the detailed description of a presently preferred embodiment.